Defense Secretary Jim Mattis says the U.S. is watching to see if the Syrian government uses chemical weapons during its offensive on the rebel-held bastion of Idlib, noting Syrian opposition fighters are not able to carry out such attacks.

“We have zero intelligence that shows the [Syrian] opposition has any chemical capability,” Mattis said. On the other hand, Mattis said there is evidence that President Bashar al-Assad has “done this before and we are watching very closely for this.”

U.S. officials have warned of a potential humanitarian catastrophe as the Syrian government moves in on the last remaining stronghold of Syrian rebels in the northwestern province of Idlib. An estimated three million people live in the area.

Stefan de Mistura, the top U.N. envoy for Syria, has also warned of a “perfect storm coming up in front of our eyes potentially.”

Assad’s government has been accused of repeatedly using chemical weapons during the seven-year-old war. Assad rejects the accusation, instead accusing rebels of using the chemical agents.

The Trump administration has twice launched airstrikes against Syria’s government in response to the chemical weapons attacks.

“Let us be clear, it remains our firm stance that if President Bashar al-Assad chooses to again use chemical weapons, the United States and its Allies will respond swiftly and appropriately,” said White House Press Secretary Sarah Sanders.

Mattis, who spoke en route to India, noted that the Islamic State group has made several “sophomoric” attempts to use chemical weapons in Iraq, but that they were “generally failures.”

“You know, a grenade with some kind of mustard agent in it or something,” Mattis said. “But nothing along the lines of what the white helmets and United Nations have seen exposed by the Assad regime’s violation of the prohibition on chemical on chemical weapons.”